Pertussis toxin suppresses carbachol-evoked cardiodepression but does not modify cardiostimulation mediated through beta1- and putative beta4-adrenoceptors in mouse left atria: no evidence for beta2- and beta3-adrenoreceptor function.

Activation of beta1-, beta2-, beta 3- and putative beta4-adrenoceptors modifies cardiac function. These receptors are usually coupled to Gs protein, but beta2- and beta3-adrenoceptors could also couple to Gi/o proteins. The mouse heart is used increasingly for studies of genetically disrupted or overexpressed proteins, including beta-adrenoceptor subtypes. We therefore investigated in contracting mouse left atria (2 Hz, 37 degrees C) if inactivation of Gi/o proteins with pertussis toxin modifies or uncovers effects mediated through beta-adrenoceptor subtypes. The negative inotropic effects of carbachol in atria exposed to catecholamine or high calcium (6.8 mmol/l) were assumed to be mediated through activation of muscarinic receptors coupled to Gi/o. We report conditions under which incubation of left atria with 200 ng/ml pertussis toxin for 24 h nearly abolished the carbachol responses. Although it has been reported that muscarinic receptor-mediated cardiodepression has an obligatory contribution of nitric oxide, the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (0.1-1 mmol/l) did not modify the negative inotropic effects of carbachol, inconsistent with an involvement of nitric oxide. The positive inotropic effects of (-)-noradrenaline and (-)-adrenaline, mediated through beta1-adrenoceptors, were not affected by pertussis toxin. (-)-Adrenaline did not cause positive inotropic effects attributable to beta2-adrenoceptor-mediation, in the presence of CGP 20712A (300 nmol/l) to block beta1-adrenoceptors, in control atria or atria pretreated with pertussis toxin. The positive inotropic effects of (-)-CGP 12177 (1 micromol/l), a compound with agonist activity at the putative beta4-adrenoceptor, were unaffected by pertussis toxin. The beta3-adrenoceptor-selective agonist BRL 37344 (1 micromol/l), in the presence of (-)-propranolol (200 nmol/l), did not cause positive or negative inotropic effects in control and pertussis toxin-treated atria. In left atria obtained from mice injected with 150 microg/kg i.p. pertussis toxin which abolished carbachol-evoked cardiode-pression, the positive inotropic effects of (-)-adrenaline were antagonised by CGP 20712A. The beta2-adrenoceptor-selective antagonist ICI 118551 (50 nmol/l) did not cause additional blockade of the effects of high (-)-adrenaline concentrations in the presence of CGP 20712A, ruling out the involvement of beta2-adrenoceptors. The results with intraparenteral PTX validate our in vitro PTX method. We conclude that inhibition of murine Gi/o proteins does not alter atrial positive inotropic effects mediated through beta1- and putative beta4-adrenoceptors and does not reveal functional beta2- and beta3-adrenoceptors.